1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM). Moreover, the present invention relates to a structure of a write wiring for writing data in a memory cell of the MRAM.
2. Description of the Related Art
Many memories for storing information in accordance with a new principle have been proposed in recent years. As one of the memories, a nonvolatile and high-speed MRAM in which magnetic memory cells constituted by an MTJ (Magnetic Tunnel Junction) device for storing information of “0” and “1” in accordance with the tunneling magneto resistive effect are arranged like a matrix is described in, for example, Roy Scheuerlein et. al “A 10-ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC 2000 Technical Digest pp. 128-129.
Data is written in an MTJ device by supplying a current to a write word line and a bit line and making the spin direction of the MTJ device parallel or antiparallel by using a magnetic field generated by the current circulating through the both lines.
It is said that the largest problem of an MRAM is reduction of a write current. An MTJ device reported at present requires a large write current value of 8 to 10 mA when the cell width ranges between 0.4 and 0.6 μm and the cell length is approx. 1.2 μm. Therefore, the MTJ device has a problem on electromigration of a wiring and a problem that a driving circuit occupies a large area.
Even in the case of a test chip of a 1K-bit-level MRAM fabricated by the present inventors by way of trial, a write current also ranges between 8 and 10 mA. To practically use the MRAM, it is indispensable to reduce the write current value to an allowable level such as 1 to 2 mA.
To rewrite data in an MTJ device, it is necessary to invert the magnetizing direction of a recording layer of the MTJ device. A switching field Hsw necessary to rewrite the magnetization information of the recording layer is approximately obtained from the following expression:Hsw=4π×Ms×t/F (Oe)where Ms denotes saturated magnetization of the recording layer, t denotes a thickness of the recording layer, and F denotes a width of the recording layer.
Reduction of the thickness of the recording layer of an MTJ device is restricted to secure the stability of the information. In the case of an MTJ device in which the width F of the recording layer is decreased to approx. 0.15 μm or less, it is necessary to increase the thickness t of the recording layer.
Even if forming the recording layer by a CoFeNi thin film and setting the thickness of the layer to 2 nm, the switching filed Hsw is intensified and a write current further increases by decreasing the width F of the recording layer and fining the MTJ device.
On the other hand, a current density to be supplied to a wiring has an upper limit. In the case of a copper wiring, the upper limit is approx. 107 A/cm2. Because the sectional area of the wiring decreases as the device is further fined, it is impossible to supply a current capable of generating the switching field Hsw necessary to invert the magnetization of the recording layer.
As a method for reducing a write current in an MRAM, it is reported in, for example, Saied Tehrani, “Magneto resistive RAM”, 2001 IEDM short coursew to use a write wiring provided with a yoke covered with a soft magnetic body made of NiFe or the like for a wiring made of Cu.
As a result of studying a case of using the write wiring provided with the yoke described in the above document through an experiment and a computer simulation, an efficient improvement effect of approx. two times ca be confirmed and a write current can be decreased to 5 mA. However, a write current of 5 mA is a limit. Therefore, the value 5 mA is far from reduction of current to 1 to 2 mA which is a target value of a write current necessary for practical use.
Moreover, as a result of quickly writing data at a short-pulse write current of approx. 50 nsec, a necessary write current fluctuates and only a reproducibility can be obtained which is greatly lower than a reproducibility 90% when writing data at a constant write current.
As described above, though a write wiring provided with a yoke is proposed which is obtained by covering the side face of the write wiring with a magnetic layer in order to reduce the write current of an MRAM, it has a write current considerably larger than a target value necessary for practical use. Moreover, as a result of quickly writing data at a short-pulse write current, problems occur that a necessary write current value fluctuates and write reproducibility is low.
Therefore, it is requested that a write current can be reduced and write reproducibility is secured without fluctuation of a write current value even when writing data at a short-pulse write current.